Coring is the process of taking a core sample from a subterranean formation. For many years, geologists in the oil and gas exploration industry have taken and analyzed core samples of potential hydrocarbon-producing formations as part of their efforts to determine the profitability of completing wells in formations from which the core samples are taken. Several types of coring techniques are used to take a core sample from a well traversing a subterranean formation including: a) conventional, (b) diamond, (c) wireline and (d) sidewall coring techniques. Before coring occurs, the borehole fluid is circulated to remove carvings and loose material. The conventional and diamond cores are taken with a core barrel and drillstring. Wireline coring is a method of coring a well without tripping in and out with the drillstring. The core is raised by the wireline in a re-tractable core barrel inside the drillstring. The cores range up to 15 feet (4.57 meters) in length and between 11/8 to 21/2 inches (2.85 to 6.35 centimeters) in diameter. In sidewall coring, a small core is taken from the sidewall of a borehole by a sidewall coring tool or gun after the well has been drilled. The cores are used to verify the lithology of formations seen on electronic logs and to sample the fluids present in the formation. Grain shattering is common in the sample. The fracturing of tight rocks tends to increase their porosity and permeability, whereas the compaction of soft rock tends to decrease their porosity and permeability.
Sidewall cores are relatively inexpensive and are used primarily in exploration wells. In these coring methods, less weight is applied to the bit during coring than during drilling, and a slower rotary speed is used in the coring process. The core is broken off in the core barrel and then retrieved and placed in trays, labeled, inspected, and described on the rig floor. The core is wrapped, sealed and sent to the laboratory. Coring is generally expensive because it involves rig time.
One problem with coring tools in exploratory wells is the lack of knowledge of the exact location of the formation of interest, or "pay zone," with respect to well depth. In many instances, drillers have unintentionally drilled completely through the pay zone or target formation without taking any core samples because the pay zone was at a slightly lesser depth than anticipated. Since logging tools can be run into a borehole before it is cased and cemented to determine the location or locations of potential pay zones, coring of these promising zones subsequent to drilling and identification thereof via open-hole logging of the borehole provides a means to verify and enhance the information on production potential provided by seismic surveys and well logs. For obvious reasons, the only economic way to take core samples from a drilled borehole is from its sidewall.
Sidewall coring tools are used to cut and retrieve several cylindrical cores from the bore hole for laboratory analysis of formation properties. These tools shoot or punch sample cups into the sidewall of the borehole perpendicular to the borehole axis and retrieve the cups when the tool is retrieved. Alternatively, coring tools have been employed which drill cores perpendicular to the borehole. Both types of tools are limited to an extremely small diameter and consequently produce very short cores which do not provide an adequate amount of formation to analyze and which may therefore not be representative of the target formation characteristics. In addition, such tools often cause damage to the formation by their operation.
Conventional sidewall coring tools usually employ only one cutting bit that may be able to cut a maximum of 50 cores. After the core is cut, each core is forced out of the bit by a ram into a core holding chamber. A spacer or marker is then dropped on top of the core to identify the sequence and the depth at which the core was removed. Although this system obtains cores in the field, there are some major problems with this system. The main problem occurs when the bit fails to obtain a core. This situation occurs while coring in a washout or if the ramming device fragmented the core during extraction. When this happens, the marker would then fall on top of the previous marker making identification of core depth difficult. Another problem arises from the use of only one bit to cut all the cores. After about 30 cores, the bit begins to wear significantly and the teeth start to clog with debris. The worn bit and debris substantially reduces the efficiency of the coring tool.
The existing sidewall coring tools do not adequately address the current needs associated with sidewall coring. U.S. Pat. No. 4,354,558 discloses a sidewall coring tool that can cut and store multiple coring samples in a single borehole run. However, only a maximum of eight cores can be stored. Furthermore, each cutting bit stores two core samples. This technique can lead to a contamination of the two samples being stored in each core barrel. U.S. Pat. No. 4,466,495 discloses a method to take a core sample and maintain the sample at the formation pressure. U.S. Pat. No. 4,449,593 describes another conventional sidewall coring tool. In this patent, the cut core sample is forced from the core barrel into a core container. This technique can have problems with indexing the core sample and with core contamination. U.S. Pat. No. 4,461,360 only describes an apparatus to extend and guide a cutting bit.
Although there are sidewall coring tools, there still remains a need for an improved coring tool that is more reliable and can take multiple core samples of various formation hardnesses in a single borehole run.